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None that I have measured. If there is any, it is very small.


I many use synthetic oil for its stable viscosity properties at high temperatures and over long mileage intervals. One would also use synthetic oil for its low temperature flow characteristics (though that is not an issue in So Cal.)
 

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DMAX2DMAX


On my 6.2, I changed rear end, engine, transmission and greased the joints, all Delvac 1 and Mobil 1. I have fairly detailed fuel records for about 29K miles (13 months). ~18k miles before ~11k miles after. I was previously running Mystic 15-40 in the engine. Throwing out a few oddballs ,Speedo cable broke, trailer etc, I went from 17.1 average to 18.4 average. Most impressive were the highs going from 18s to 18.9. 19.9, 21, 21.4,22. {21 years old, 350,000 miles and still capable of 20-22mpg!}

I didn’t get as drastic results with my GMC Gasser with oil change only. I didn’t have as much of a baseline (~2k), but appears to have added .2-.4 mpg.


PM and I'll send you a spread sheet of the 6.2 results.
 

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I changed my entire driveline -- tranny, front & rear diffs, and tcase -- to synthetic the same time I did the engine oil. With all that I got almost a 2 MPG improvement. And the tranny runs a bit cooler too, which is a good thing.
 
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